1. Field of the Invention
The present invention relates to a process for the preparation of para-benzoquinone by oxidizing phenol with oxygen or with a gas in which oxygen is present.
2. Discussion of the Prior Art
Para-benzoquinone, which will hereinafter be denoted more simply, as "benzoquinone" or "p-benzoquinone", is an organic product of particular importance industrially because it affords, by hydrogenation, hydroquinone. The latter compound is widely used in the photographic industry.
3. Description of the Prior Art
Numerous processes for the preparation of hydroquinone from phenol have been previously proposed; these processes essentially involve the hydroxylation of phenol with hydrogen peroxide itself or with organic peracids such as peracetic and performic acids which are generated in situ from hydrogen peroxide and the carboxylic acid. In all cases, these processes lead to the concomitant formation of hydroquinone and pyrocatechol, the latter product generally being formed in preponderant amounts. Certain of these processes of hydroxylation with hydrogen peroxide have shown themselves to be of great value and are the subject of industrial exploitation for the production of diphenols. Nevertheless, industry is seeking a process which makes it possible to obtain hydroquinone from phenol selectively, while limiting or even totally avoiding the formation of the other diphenols and especially of pyrocatechol. A means of resolving this problem would be the perfection of a process for the selective oxidation of phenol to benzoquinone with molecular oxygen or with a gas in which oxygen is present.
Thus, the French patent application published under No. 2,245,602 describes a process for the preparation of benzoquinones from various phenols, and especially of benzoquinone from phenol, by oxidation with molecular oxygen or with a gas in which oxygen is present (for example, air), in the presence of a catalyst comprising copper and a chloride, bromide, iodide, thiocyanate, cyanate or cyanide ligand, in a polar solvent. Although copper metal can be employed under conditions which enable it to be oxidized to cuprous or cupric ions, it is generally copper salts, especially cuprous or cupric halides and, in particular, cupric chloride, which are used. U.S. Pat. No. 3,987,068 has proposed an analogous process by which the reaction is carried out in the presence of a copper salt in a nitrile which forms a complex with the copper salt. Although these processes generally ensure a good degree of conversion of the phenol and industrially valuable yields of benzoquinone, it has been found that when the process is carried out in a reaction apparatus which is not inert towards the reaction medium--i.e., an apparatus made of steel or iron--its walls are rapidly attacked to such an extent that it is virtually impossible to envisage the use of such a process in industry. On the other hand, it has been found that, if equipment is used which is inert towards the reaction medium, such as, for example, reactors made of enamelled metal or of tantalum, no oxidation of the phenol occurs when the reaction is carried out in the presence of copper-II ions.